Why does Quaternion.rotation_difference() always return 12.666471 for x-rotation?
The issue is the test data, the vector v = Vector(( 1.0 , 0.0 , 0.0 )) lies exactly on the x-axis and a rotation around x has absolutely no effect, therefore the information is lost.

In order to get the rotation from v to vrot it is requiresd to invoke rotation_difference() on v insteadof vrot.